1. Field of the Invention
The present invention relates to a magneto-optical disc system for performing recording and/or reproduction of an information signal and to a magneto-optical disc used in such system.
2. Description of the Prior Art
A magneto-optical disc system utilizing a magneto-optical effect for performing recording in and/or reproduction from a magneto-optical disc is shown in FIG. 1. A magneto-optical disc system shown in FIG. 1 is arranged such that when a disc-like one-sided magneto-optical recording medium 50 is mounted, an optical system including a laser unit 58, an objective lens 59, and the like is arranged above the one-sided magneto-optical recording medium 50, and a magnetic field generation unit 60 serving as a magnetic system is arranged below the one-sided magneto-optical recording medium 50. A numerical aperture (to be referred to as NA hereinafter) of the objective lens 59 is set to fall within the range of 0.50 to 0.53.
In order to drive the optical system in focusing and tracking directions with respect to the magneto-optical recording medium 50, a drive system (not shown) is provided. Another drive system (not shown) is provided to drive the magnetic system in a radial direction on the recording medium 50 and in the tracking direction.
In this magneto-optical disc system, a magnetic field modulation scheme is employed to perform recording. In this magnetic field modulation scheme, high-speed reverse control of a magnetic field must be performed in accordance with an information signal to be recorded. For this reason, a sufficiently large excitation current cannot be obtained. The intensity of the magnetic field generated by the magnetic field generation unit is limited. The magnetic field generation unit 60 is located near a magnetic recording layer 53, to be described below, in the one-sided magneto-optical recording medium 50. An overwrite operation can be performed according to the magnetic field modulation scheme.
The one-sided magneto-optical recording medium 50 is formed as follows. A dielectric layer 52, a magnetic recording layer 53 having a large magneto-optical effect and made of, for example, a rare earth element-transition metal alloy amorphous thin film, a dielectric film 54, a reflecting layer 55, and a protective cover 56 are sequentially stacked on one surface of a light-transmitting transparent substrate 51 made of, for example, polycarbonate. The transparent substrate 51 has a predetermined thickness t.sub.1. The thickness of a conventional substrate is set to be 1.2 mm.
An operation of this magneto-optical disc system will be described below.
The one-sided magneto-optical recording medium 50 is placed on and driven by a rotary table (not shown), and a magnetic field is applied from the magnetic field generation unit 60 to the magnetic recording layer 53 of the one-sided magneto-optical recording medium 50. High-speed reverse control of the magnetic field to be applied is performed on the basis of an information signal to be recorded. A laser beam emitted from the laser unit 58 is focused through the objective lens 59 on the magnetic recording layer 53 applied with this magnetic field. A change in direction of magnetization occurs in a region of the magnetic recording layer 53 irradiated with the focused laser beam in accordance with the direction of the magnetic field applied from the magnetic field generation unit 60. Therefore, an overwrite operation of an information signal can be performed in real time.
In order to obtain a more compact magneto-optical pickup, which is constituted by the optical system, the magnetic system, and the drive systems for driving the optical and magnetic systems, it is assumed that the optical system may be formed integrally with the magnetic system, with both systems being arranged on one side of the magneto-optical recording medium. More specifically, the magnetic field generation unit 60 of FIG. 1 is located on the same side as the objective lens 59, that is, on the side of the transparent substrate 51. Nevertheless, because a distance between the magnetic field generation unit 60 and the magnetic recording layer 53 then becomes larger than before, a sufficiently high magnetic field cannot be applied to the magnetic recording layer 53.
Along with an increase in information volume in recent years, a two-sided magneto-optical recording medium has been developed, in which the magnetic recording layers are formed respectively on both surfaces of one magneto-optical recording medium so as to be capable of recording information signal son each of the surfaces.
In order to perform recording in and/or reproduction from such a two-sided magneto-optical recording medium it is very difficult to apply a sufficiently high magnetic field to each of the magnetic recording layers by means of the magneto-optical pickup of FIG. 1, constituted by the optical and magnetic systems. The reason for this is that in the magnetic field generation unit using the magnetic field modulation scheme, a high-frequency current corresponding to a high-frequency data signal representing the information to be recorded must be supplied to an electromagnetic coil. In that situation, it tends to become more and more difficult for the current to flow through the electromagnetic coil as the frequency of the current increases, so that the strength of the generated magnetic field is limited. In addition, another reason is that the distance between the magnetic field generation unit and each magnetic recording layer is rather large. Therefore, in state-of-the-art techniques it is very difficult to perform two-side optomagnetic recording under the magnetic field modulation scheme.
In order to cope with an increase in information volume, more information signals must be recorded in the recording portion such as the magnetic recording layer of a magneto-optical disc.